COAST GUARD CUTS SURFACE SEARCH; INQUIRY GOES SECRET

The Coast Guard concluded its ocean search Friday for surface debris from the space shuttle Challenger, while a presidential commission investigating the disaster met behind closed doors in Washington.

And the USS Preserver, a 214-foot Navy salvage and rescue ship, headed out to sea with about 22 divers aboard to scour the depths for underwater remnants of Challenger, which exploded 74 seconds after launch on Jan. 28 and plunged into the Atlantic off the Florida coast, killing its seven crew members.

Recovery teams and ships concentrated Friday on bringing up what is believed to be the wreckage of the shuttle`s two solid rocket boosters, considered to be major elements in the investigation to determine what happened to Challenger.

''They`re picking up signals,'' said NASA spokesman James Mizell, referring to the sonar soundings that showed debris scattered over a wide undersea area as the search proceeded amid fog and heavy seas. He said the debris is where radar followed the boosters into the sea.

Recovering the boosters may tell investigators what caused flame to shoot from the right booster. The errant plume of flame is believed to have triggered the explosion.

(Defense Department sources told the Washington Post they received word from NASA that the space agency is close to pinpointing the cause of the shuttle explosion, United Press International reported Saturday.

(The Air Force plans to go ahead with preparations for the first space shuttle launch from Vandenberg Air Force Base, Post reported.

(''It was not as serious as we once thought it would be,'' a Pentagon official said. ''The shuttle may be back flying in six months.''

(The first shuttle launch from Vandenberg, in California, is tentatively scheduled for mid-July.

(A Defense Department official told the Post that NASA has ''infrared film'' that links the cause of the Challenger explosion to the solid rocket booster.

(The West Coast flight could be pushed forward if the sole problem turns out to be the rocket booster since the Vandenberg shuttle will be powered by two new solid-fuel booster rockets, the Post quoted sources as saying.)

Meanwhile, NASA officials called employees together several times Friday at the Kennedy Space Center in an attempt to boost sagging morale.

In a series of four pep talks given to the space center`s 16,000 workers, center director Dick Smith and Rep. Bill Nelson (D., Fla.), who flew on a shuttle mission last month, cautioned employees against despair and said that the shuttle program would press on.

''I ask each and every one of you not to be discouraged,'' Smith told a group of workers who assembled Friday morning on the same site where dignitaries and teachers had been seated to watch Challenger`s tragic launch. ''I am convinced that we will be flying again, probably sooner than we think now.''

In an apparent reference to questions about human error and lax safety precautions that have arisen since the disaster, Smith added: ''The meetings we had, the weather problems we had, the discussions we had--there was nothing to lead us to believe that we should not attempt to launch.''

''A lot of American people were shocked because they didn`t realize the risks of going into the hostile environment of space,'' Nelson said. ''But I can tell you that every astronaut who gets into that orbiter understands the risk. They don`t expect something as cataclysmic as what happened, but they understand the risk.''

Meanwhile, the commission appointed by President Reagan to investigate the tragedy moved its hearings behind closed doors Friday.

White House spokesman Mark Weinberg said only that the panel was reviewing classified information in its closed session. Other sources, speaking privately, said members were hearing about Defense Department participation in the shuttle program, which has involved deployment of several Pentagon payloads.

In another development, White House spokesman Larry Speakes said that James Beggs, on leave as NASA administrator since his indictment on fraud charges, may resign from the post.

Beggs has been on indefinite paid leave since his indictment Dec. 2 on fraud charges unrelated to his space duties.